The present invention relates generally to electronic circuits which drive a digital display, and relates more specifically to a novel electronic circuit which adequately rejects noise and provides that a high voltage power source can power a light emitting diode without using a transformer.
Many modem household electronic devices and appliances include digital displays for communicating information and/or for allowing someone to operate the device or appliance. Examples of such devices and appliances include digital alarm clocks, video cassette recorders and ovens. One type of digital display which is often incorporated within these household devices and appliances is a light emitting diode. Before fairly recently, the use of a light emitting diode as a digital display has been reserved for those household devices and appliances which would not inherently provide that the light emitting diode be subjected to high temperatures. In other words, while it has long been possible to incorporate a light emitting diode within the electronic circuitry of devices such as common digital alarm clocks, it has not long been possible to incorporate a light emitting diode within the electronic circuitry of, for example, a household oven. However, manufacturers of light emitting diodes recently have been able to produce light emitting diodes having higher temperature ratings. Consequently, it is now possible to incorporate a light emitting diode within the electronic circuitry of devices and appliances which will necessarily subject the light emitting diode to high temperatures. For example, it is now possible to incorporate a light emitting diode within the electronic circuitry of a household oven, where the light emitting diode may be exposed to temperatures as high as 105 degrees Celsius.
The typical household electrical outlet in the United States provides AC voltage of about 120 Volts at a frequency of about 60 Hz. The typical household electrical outlet in most foreign countries provides AC current at an even higher voltage. Because a light emitting diode can handle only a fraction of the voltage which an electrical outlet provides, it is imperative to provide a substantial voltage drop from the electrical outlet to the light emitting diode. To this end, electronic circuits which incorporate a light emitting diode include one or more static power transformers (each of which consists of two or more tightly coupled inductors). Transformers are used instead of resistors to drop the voltage because light emitting diodes generally require too much current to light. While the transformers get their primary power supply from the AC electrical outlet and then do, in fact, successfully provide a suitable voltage output to the light emitting diode, transformers are relatively expensive, and therefore raise the overall cost of the electronic circuit.
To avoid having to incorporate a transformer within an electronic circuit which provides a digital display, it has been possible to utilize a Vacuum Fluorescent Display in place of a light emitting diode. A Vacuum Fluorescent Display requires less current than a light emitting diode, and still requires that a voltage drop be provided from the electrical outlet. Therefore, one or more voltage dropping resistors are typically provided in connection with the Vacuum Fluorescent Display. While the one or more voltage dropping resistors provide the necessary voltage drop, a resistor elevates in temperature as it drops voltage, and it is almost always desirable to minimize the internal heat generated within an electronic circuit. This is especially true when the electronic circuit is already within an environment which necessarily subjects the electronic circuit to extreme external temperatures, such as when the electronic circuit is incorporated within a household oven, as explained above.
Another way to avoid having to incorporate a transformer within an electronic circuit which provides a digital display is to utilize a duplex display and a clock chip. This type of circuit is disclosed in U.S. Pat. No. 4,697,930, and is used within some digital alarm clocks today. Within this type of circuit, alternate phases of the AC power line are current limited and half wave rectified and become the current source to run the light emitting diode digital display. The current does not go through a bridge rectifier or a regulator. However, the less expensive the light emitting diodes, the more current that is needed to brightly drive the light emitting diodes. Additionally, the type of circuit which utilizes a duplex display and a clock chip offers noise rejection which is substandard within many applications. As a result, it often remains more practical to instead use a transformer in order to obtain the correct current and voltage drop.
Therefore, in the past, it has not been possible to provide that a high voltage power source, such as a household electrical outlet, can power a light emitting diode without having to include at least one transformer in order to offer adequate noise rejection. Having to include a transformer raises the overall cost of the circuit.
Additionally, it has not been possible to provide an electronic circuit which can drive a digital display, and offer adequate noise rejection, without generating substantial heat within the electronic circuit.
For the foregoing reasons, there is a need for the present invention, and the present invention is directed to eliminate the problems encountered heretofore.